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A SUPPLEMENTAL FUSION-FISSION HYBRID PATH TO FUSION POWER DEVELOPMENT Presentation to EPRI Workshop on Fusion Energy Assessment Palo Alto, CA 7/21/2011 By Weston M. Stacey For the Georgia Tech SABR Design Team

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Outline Fusion R&D for electrical power production. What are fusion-fission hybrids (FFHs) & what is their raison d’etre? What is the time scale for developing the fusion neutron source for a FFH? The SABR conceptual design for a FFH burner reactor. SABR transmutation fuel cycle studies. SABR (preliminary) dynamic safety studies. R&D requirements for developing fusion power, with and without FFH. Schedules for developing fusion power, with and without FFH. Some technical issues with combining fusion and fission. Three recommendations.

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Assessment of R&D Needed for Fusion Power Production 4 Levels of Performance Questions 1.What must be done to achieve the required level of individual physics and technology performance parameters? (physics and technology experiments) 2.What further must be done to achieve the required levels of all the different individual physics and technology performance parameters simultaneously? (component test facilities & experimental reactors, e.g. ITER) 3.What further must be done to achieve the required level of all the individual physics and technology performance parameters simultaneously and reliably over long periods of continuous operation? (advanced physics experiments, component test facilities & demonstration reactors) 4.What further must be done to demonstrate the economic competitiveness of the power that will be produced?(prototype reactors)

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Status of Magnetic Fusion R&D 1.The tokamak is the leading plasma physics confinement concept.  ~100 tokamaks worldwide since 1957.  Physics performance parameters achieved at or near lower limit of reactor relevance.  Large, world-wide physics & technology programs supporting ITER (initial operation 2019).  ITER will achieve reactor-relevant physics and technology parameters simultaneously, produce 500 MWth and investigate very long-pulse operation. 2.Many other confinement concepts (e.g. mirror, bumpy torus) have fallen by the wayside or remain on the backburner. 3.A few other confinement concepts (e.g. stellarator, spherical torus) have some attractive features, which justifies their continued development. However, the performance parameters are at least 1-2 orders of magnitude below what is required for a power reactor, and at least 25 years would be required to advance any other concept to the present tokamak level. 4.Plasma support technology (SC magnets, heating, fueling, vacuum, etc.) for the tokamak is at the reactor-relevant level, due to the large ITER R&D effort. 5.Fusion nuclear technology (tritium production, recovery and processing) has had a low priority within fusion R&D. ITER will test fusion tritium breeding blanket modules. 6.The continued lack of a radiation damage resistant structural material would greatly complicate fusion experiments beyond the ITER level (e.g. DEMO) and might make a fusion reactor uneconomical, if not altogether impractical.

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THE FUSION-FISSION HYBRID REACTOR What is it? Mission? Rationale? Choice of technologies?

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The Fusion-Fission Hybrid What is it? A Fusion-Fission Hybrid (FFH) is a sub-critical fission reactor with a variable strength fusion neutron source. Mission? Supporting the sustainable expansion of nuclear power in the USA and worldwide by helping to close the nuclear fuel cycle.

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SUSTAINABLE NUCLEAR POWER EXPANSION The present ‘once-through’ LWR fuel cycle utilizes < 1% of the potential uranium fuel resource and leaves a substantial amount of long-term radioactive transuranics (TRU) in the spent nuclear fuel. The TRU produced by the present USA LWR fleet will require a new Yucca Mountain HLWR every 30 years, and a significant expansion of nuclear power would require new HLWRs even more frequently. A significant expansion of nuclear power worldwide would deplete the known uranium supply within 50 years at the present <1% utilization. Fast ‘burner’ reactors can in principle solve the spent fuel accumulation problem by fissioning the transuranics in spent nuclear fuel, thus reducing the number of HLWRs needed to store them, while at the same time utilizing more of the uranium energy content. Fast ‘breeder’ reactors can in principle solve the uranium fuel supply issue by transmuting U238 into fissionable (in LWRs and fast reactors) transuranics (plutonium and the higher ‘minor actinides’), leading to the utilization of >90% of the potential energy content of uranium. Fast reactors can not be fueled entirely with transuranics because the reactivity safety margin to prompt critical would be too small, and the requirement to remain critical requires periodic removal and reprocessing of the fuel. Operating fast reactors subcritical with a variable-strength fusion neutron source can solve both of these problems, resulting in fewer fast burner reactors and fewer HLWRs.

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Rationale for FFH Fast Burner Reactors Fast Burner reactors could dramatically reduce the required number of high-level waste repositories by fissioning the transuranics in LWR SNF. The potential advantages of FFH burner reactors over critical burner reactors are: 1) fewer reprocessing steps, hence fewer reprocessing facilities and HLWR repositories a —no criticality constraint, so the TRU fuel can remain in the FFH for deeper burnup to the radiation damage limit. 2) larger LWR support ratio---FFH can be fueled with 100% TRU, since sub-criticality provides a large reactivity safety margin to prompt critical, so fewer burner reactors would be needed. a separation of transuranics from fission products is not perfect, and a small fraction of the TRU will go with the fission products to the HLWR on each reprocessing.

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Choice of Fission Technologies for FFH Fast Burner Reactor Sodium-cooled fast reactor is the most developed burner reactor technology, and most of the world-wide fast reactor R&D is being devoted to it (deploy 15-20yr). 1.The metal-fuel fast reactor (IFR) and associated pyroprocessing separation and actinide fuel fabrication technologies are the most highly developed in the USA. The IFR is passively safe against LOCA & LOHSA. The IFR fuel cycle is proliferation resistant. 2.The sodium-cooled, oxide fuel FR with aqueous separation technologies are highly developed in France, Russia, Japan and the USA. Gas-cooled fast reactor is a much less developed backup technology. 1.With oxide fuel and aqueous reprocessing. 2.With TRISO fuel (burn and bury). Radiation damage would limit TRISO in fast flux, and it is probably not possible to reprocess. Other liquid metal coolants, Pb, Pb-Li, Li. Molten salt fuel would simplify refueling, but there are issues. (Molten salt coolant only?)

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Choice of Fusion Technologies for the FFH Fast Burner Reactor The tokamak is the most developed fusion neutron source technology, most of the world-wide fusion physics and technology R&D is being devoted to it, and ITER will demonstrate much of the physics and technology performance needed for a FFH (deploy 20-25 yr). Other magnetic confinement concepts promise some advantages relative to the tokamak, but their choice for a FFH would require a massive redirection of the fusion R&D program (not presently justified by their performance). 1.Stellarator, spherical torus, etc. are at least 25 years behind the tokamak in physics and technology (deploy 40-50 yr). 2.Mirror could probably be deployed in 20-25 years, but would require redirection of the fusion R&D program into a dead-end technology that would not lead to a power reactor.

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SABR FFH DESIGN APPROACH 1.Use insofar as possible the physics and technologies, and adapt the designs, that have been developed for the Integral Fast Reactor (IFR) and the International Thermonuclear Experimental Reactor (ITER). The successful operation of an IFR and associated fuel pyroprocessing and fabrication technologies will prototype the fission physics and technologies. The successful operation of ITER and its blanket test program will prototype the fusion physics and technologies. 2.Be conservative insofar as possible. Modest plasma, power density, etc. performance parameters. Adapt IFR and ITER component designs, and use IFR and ITER design guidelines on stress margins, structure fractions, etc. Use conservative 99% actinide—fission product separation efficiency.

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FUEL CYCLE CONCLUSIONS SABR FFH BURNER REACTORS A SABR TRU-burner reactor would be able to burn all of the TRU from 3 LWRs of the same power. A nuclear fleet of 75% LWRs (% nuclear electric power) and 25% SABR TRU-burner reactors would reduce geological repository requirements by a factor of 10 relative to a nuclear fleet of 100% LWRs. A SABR MA-burner reactor would be able to burn all of the MA from 25 LWRs of the same power, while setting aside Pu for future fast reactor fuel. A nuclear fleet of 96% LWRs and 4% SABR MA-burners would reduce HLWR needs by a factor of 10.

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Fusion Technology Advances Beyond ITER FFH must operate with moderately higher surface heat and neutron fluxes and with much higher reliability than ITER. PROTODEMO must operate with significantly higher surface heat and neutron fluxes and with higher reliability than ITER. PROTODEMO and FFH would have similar magnetic field, plasma heating, tritium breeding and other fusion technologies. PROTODEMO and FFH would have a similar requirement for a radiation-resistant structural material to 200 dpa.

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INTEGRATION OF FUSION & FISSION TECHNOLOGIES IS NEEDED FOR FFH For Na, or any other liquid metal coolant, the magnetic field creates heat removal challenges (e.g. MHD pressure drop, flow redistribution). Coating of metal surfaces with electrical insulation is one possible solution. This is also an issue for a PROTODEMO with liquid Li or Li-Pb. Refueling is greatly complicated by the tokamak geometry, but then so is remote maintenance of the tokamak itself, which is being dealt with in ITER and must be dealt with in any tokamak reactor. However, redesign of fuel assemblies to facilitate remote fueling in tokamak geometry may be necessary. The fusion plasma and plasma heating systems constitute additional energy sources that conceivably could lead to reactor accidents. On the other hand, the safety margin to prompt critical is orders of magnitude larger in SABR than in a critical reactor, and simply turning off the plasma heating power would shut the reactor down to the decay heat level in seconds. Etc.

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PROs & CONs of Supplemental FFH Path of Fusion Power Development Fusion would be used to help meet the USA energy needs at an earlier date than is possible with ‘pure’ fusion power reactors. This, in turn, would increase the technology development and operating experience needed to develop economical fusion power reactors. FFHs would support (may be necessary for) the full expansion of sustainable nuclear power in the USA and the world. An FFH will be more complex and more expensive than either a Fast Reactor (critical) or a Fusion Reactor. However, a nuclear fleet with FFHs and LWRs should require fewer burner reactors, reprocessing plants and HLWRs than a similar fleet with critical Fast Burner Reactors.

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RECOMMENDATIONS Perform an in-depth conceptual design of the burner reactor-neutron source- reprocessing-repository system to determine if it is technically feasible to deploy a SABR FFH Advanced Burner Reactor within 25 years and identify needed R&D. Perform comparative dynamic safety and fuel cycle studies of critical and sub- critical ABRs to quantify any transmutation performance advantages of a SABR because of the relaxation of the criticality constraint and the much larger reactivity margin of safety to prompt critical. Perform comparative systems and scenario studies* to evaluate the cost- effectiveness of various combinations of Critical, FFH and ADS Advanced Burner Reactors disposing of the legacy spent fuel TRU and the spent fuel TRU that will be produced by an expanding US LWR fleet. The cost of HLWRs and fuel separation and refabrication facilities, as well as the cost of the burner reactors, should be taken into account. * Small studies ongoing at ANL and KIT.

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The Issues to be Studied for the FFH Burner Reactor System Is a FFH Burner Reactor Technically Feasible and on what timescale? A detailed conceptual design study of an FFH Burner Reactor and the fuel reprocessing/ refabrication system should be performed to identify: a) the readiness and technical feasibility issues of the separate fusion, nuclear and fuel reprocessing/refabricating technologies; and b) the technical feasibility and safety issues of integrating fusion and nuclear technologies in a FFH burner reactor. This study should involve experts in all physics and engineering aspects of a FFH system: a) fusion; b) fast reactors; c) materials; d) fuel reprocessing/refabrication; e) high- level radioactive waste (HLW) repository; etc. The study should focus first on the most advanced technologies in each area; e.g. the tokamak fusion system, the sodium-cooled fast reactor system. Is a FFH Burner Reactor needed for dealing with the accumulating inventory of spent nuclear fuel (SNF)discharged from LWRs? First, dynamic safety and fuel cycle analyses should be performed to quantify the advantages in transmutation performance in a FFH that result from the larger reactivity margin to prompt critical and the relaxation of the criticality constraint. Then, a comparative systems study of several scenarios for permanent disposal of the accumulating SNF inventory should be performed, under different assumptions regarding the future expansion of nuclear power. The scenarios should include: a) burying SNF in geological HLW repositories without further reprocessing; b) burying SNF in geological HLW repositories after separating out the uranium; c) reprocessing SNF to remove the transuranics for recycling in a mixture of critical and FFH burner reactors (0-100% FFH) and burying only the fission products and trace transuranics remaining after reprocessing; d) scenario “c” but with the plutonium set aside to fuel future fast breeder reactors (FFH or critical) and only the “minor actinides” recycled; e) scenarios (c) and (d) but with pre-recycle in LWRs; etc. Figures of merit would be: a) cost of overall systems; b) long-time radioactive hazard potential; c) long-time proliferation resistance; etc. What additional R&D is needed for a FFH Burner Reactor in addition to the R&D needed to develop the fast reactor and the fusion neutron source technologies? This information should be developed in the conceptual design study identified above.

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Relation Between Fusion and Fission Power Sub-critical operation increases fuel residence time in Burner Reactor before reprocessing is necessary As k decreases due to fuel burnup, Pfus can be increased to compensate and maintain Pfis constant. Thus, sub-critical operation enables fuel burnup to the radiation damage limit before it must be removed from the reactor for reprocessing.

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Sub-critical operation provides a larger margin of safety against accidental reactivity insertions that could cause prompt critical power excursions.